The general type of pressure swing adsorption apparatus with which this invention is concerned is set forth in U.S. Pat. Nos. 4,802,899 and 5,531,807. Generically, a pressure swing adsorption apparatus includes a pair of adsorbers each having a fixed bed of adsorbent material disposed between an inlet and an outlet for accommodating a flow of gas therethrough. The adsorbent material of each adsorber fractionates at least one constituent gas from a gaseous mixture when the gaseous mixture from a feed stream is sequentially directed through the inlet and the outlet of one adsorber. That direction is called a co-current direction. While one adsorber performs adsorption the other adsorber is simultaneously purged of adsorbed constituent with product gas that is withdrawn from the outlet of one adsorber and directed through the outlet of the other adsorber to the inlet of the other adsorber. That direction is called a counter-current direction. Once the other adsorber is purged, the gaseous mixture is directed into the inlet of the other adsorber so that the other adsorber performs adsorption while the one adsorber is simultaneously purged of adsorbed constituent with product gas withdrawn from the outlet of the other adsorber.
Highly concentrated product gas is attained by controlling the zones of the adsorber. A conventional-adsorber contains four zones as illustrated in FIG. 1A. When compressed air enters the adsorber 2 through the input 4, the air contains about 78% nitrogen, 21% oxygen, 0.9% argon, and a variable amount of water. The first zone or water zone 6, captures the water. The second zone 8 adsorbs the fractionated nitrogen and a lesser degree oxygen. The third zone or mass transfer zone ("MTZ") 10 comprises a varied concentration of product oxygen gas that increases in concentration in relation to the distance from the second zone 8 by capturing more fractionated nitrogen. The fourth zone 12 comprises the high concentrated product oxygen gas and extends to the terminal end 14 of the adsorber 2. Controlling a pressure swing adsorption apparatus to attain a product gas having a relatively high concentration of a particular gas, like oxygen, is conventional.
The high concentrated product oxygen gas is achieved by stabilizing the water zone 6 near the input 4 of the adsorber 2 and maintaining the MTZ 10 as a sharp demarcation between the second and fourth zones 8, 12. These zones 6, 10 are stabilized and maintained by directing certain quantities of product oxygen gas counter-currently through the adsorber 2.
Some applications, however, require a lower concentration of product oxygen gas. Controlling the apparatus to produce a product oxygen gas having a desired and specific concentration and flow rate in which these characteristics are independent of each other is an object of this present invention.
In the past, there have been proposals to attain the desired concentration of product oxygen gas in a pressure swing adsorption apparatus. One such proposal entails blending the gaseous air mixture from the feed stream with the high concentrated product oxygen gas. Such a proposal has numerous adverse effects on the product oxygen gas. One such adverse affect is that the concentration of product oxygen gas is dependent upon the flow of the gaseous mixture. Thus, when the flow of gaseous mixture is disturbed the concentration of product gas is also affected. Another adverse effect is that the gaseous mixture is compressed and therefore potentially "wet." This "wet" gaseous mixture when blended with product gas can cause condensation in the product oxygen gas which may not be desired.
Another proposal would overdraw the pressure swing adsorption apparatus. Overdrawing occurs when the product oxygen gas is drawn from adsorber 2 at a high flow rate. The high flow rate moves and extends first zone 6, second zone 8, and MTZ 10. In particular, the MTZ 10 moves and extends through and beyond the terminal end 14 of the adsorber 2 as illustrated in FIG. 1B. When the MTZ 10 reaches and extends through the terminal end 14 the product oxygen gas is withdrawn at an unknown and uncontrollable lower concentration. Overdrawing has a number of disadvantages. First, the efficiency of the apparatus is compromised because it is difficult to stabilize the water zone 6 when the MTZ 10 moves and extends through to the terminal end 14. Second, it is difficult to control the MTZ 10 when the MTZ 10 is at the terminal end 14, which in turn makes it difficult to control the concentration range of the product oxygen gas generated from the adsorber 2 as illustrated in FIG. 1C. Third, it is difficult to duplicate for a particular concentration. Lastly, the flow rate and the concentration of the product gas are intrinsically dependent upon each other since the flow rate controls the concentration level of product gas.
In yet another proposal disclosed in U.S. Pat. No. 5,531,807, a pressure swing adsorption apparatus comprises a flow controller that regulates the flow rate of the product gas. It was proposed that if the flow controller discharges the high concentrated product gas at about 7 liters per minute, the product gas would decrease in concentration. This proposal, however, did not disclose how to control the concentration level of the product gas.
It is an object of the present invention to provide a new and improved pressure swing adsorption apparatus (oxygen concentrator) wherein the apparatus can be set to attain variously desired concentrations and flow rate of product oxygen gas wherein the flow rate and concentration are independent of each other.